1. Field of the Invention
This invention relates to the field of electrographic recording and, more particularly, to apparatus for developing electrostatic charge patterns.
2. The Prior Art
In the electrographic recording process, a latent electrostatic charge pattern borne by a recording element (e.g. a dielectric or photoconductive member) is rendered visible by the application of thermoplastic toner particles. Typically, such toner particles form part of a development mix which may also include magnetic or magentizable carrier particles to which the toner particles adhere via triboelectric forces. When such a development mix is applied to an electrostatic charge pattern, the electrostatic forces associated with the pattern act to strip the toner particles from the carrier and apply them to the charged areas of the recording element.
It is conventional in the art to apply electrographic developer to an electrostatic charge pattern with a magnetic brush applicator. Such an applicator typically comprises a cylindrical sleeve of non-magnetic material (e.g. stainless steel, plated plastics) having a magnetic field-producing core piece positioned therein. Lines of force from the magnetic core piece penetrate the non-magnetic sleeve and thereby draw the magnetically attractive development mix to the outer surface of the sleeve. As the core piece and sleeve rotate relative to one another, the developer is transported from a reservoir to a position in which it contacts the electrostatic charge pattern.
In the commonly assigned U.S. Pat. No. 4,473,029, issued to Fritz et al, there is disclosed an electrographic development system comprising a magnetic brush applicator and a developer comprising magnetically "hard" carrier particles. The brush's magnetic core is rotated at a high rate of speed (e.g. 1500 RPM) which produces rapid changes in polarity of the magnetic field at the brush's outer surface. These polarity changes cause the hard magnetic particles to continuously flip-flop, end-over-end, in attempting to align themselves with the instantaneous magnetic field. Such flip-flopping of particles acts to continuously churn the developer during transport by the brush. While this churning effect is highly desirable from the standpoint that it serves to continuously present "fresh" developer to the outermost portion of the brush nap, it does present certain difficulties, as explained below.
Not to be confused with any prior art observation, we have observed a significant increase in developer temperature in development systems of the above-mentioned type. We attribute this temperature increase, which may be as much as 40.degree. C., to frictional heating which occurs while the developer is being transported and, hence, churned by the brush. While such a temperature increase may have little or no effect on some toners, it has been found to have a marked adverse affect on toners having relatively low glass transition temperatures, such as those used in high speed electrographic copiers. Such toners tend to become sticky or tacky when subjected to this amount of heating and, as this occurs, the development process gradually deteriorates.
The need to control developer temperature in the electrographic reproduction process has been recognized before. For example, U.S. Pat. No. 4,112,870, issued to Extra et al discloses an electrographic magnetic brush system in which the bottom of the developer reservoir is made "double-walled" to provide a cooling space through which a stream of air can be maintained. The purpose of this air stream is to conduct heat away from the reservoir and thereby maintain the developer temperature within an acceptable range. In this case, the noted increase in developer temperature was caused by the frictional heat produced by the action of mixing augers positioned at the bottom of the developer reservoir. While a double-walled reservoir may be useful in cooling developer in the vicinity of mixing augers, it would not be effective in dissipating the heat generated by the churning action on the surface of the brush.